Monochromators are optical devices used transmit a narrow band of wavelengths of light or other radiation. In X-ray, gamma-ray and neutron optics, crystal monochromators are used to define wave conditions (e.g., to select a defined wavelength of radiation to be used).
Perfect crystals exhibit a high reflectivity of close to 100% but a very low mosaicity. In particular, perfect crystals typically have a mosaicity of only a few (e.g., 1-2) arcseconds.
The known phenomenon of X-ray and slow neutron Bragg diffraction from perfect single crystals (single crystals with high structure quality), particularly, Si, Ge, SiO2, is employed for monochromatization of these types of nuclear radiation. The perfect crystals reflect the radiation in a very narrow angular range of several arcseconds at a specific Bragg angle. Thus, despite a very high level of spectral resolution achieved with the use of diffraction from perfect crystals, there is a drastic decrease of reflected nuclear radiation intensity relative to incident radiation. This low reflectivity makes employment of perfect single crystal monochromators unacceptable for many applications in slow neutron optics.
A neutron beam typically has a divergence of about 10-20 arcminutes. Accordingly, when perfect crystals are used as monochromators for neutron optics, only a small fraction of the total neutron beam is reflected off of the monochromator. For example, a perfect crystal with a mosaicity of 2 arcseconds would only reflect about 1.7% of the total flux of the neutron beam assuming a 100% reflectivity. Accordingly, perfect crystals are not acceptable for monochromators in many applications.
For over 50 years attempts have been made by many organizations to create Ge crystal monochromators that have both a high mosaicity and a slow neutron high reflectivity. However, theory and empirical study shows that changes to a crystal structure that cause increases in mosaicity also cause decreases in reflectivity. The imperfect crystal diffraction theory states that increases in mosaicity are accompanied by corresponding decreases in peak reflectivity. This phenomenon has also been observed experimentally. For example, this phenomenon was shown for mosaic Ge crystals in Kozhukh, Low-temperature conduction in germanium with disorder caused by extended defects, J. Phys. Condens. Matter 5, pp. 2351-2376 (1993). FIG. 22 of Low-temperature conduction in germanium with disorder caused by extended defects shows a peak reflectivity of about 40% for a Ge crystal with a mosaicity of about 20-30 arcminutes. To date all attempts to create Ge slow neutron crystal monochromators with both a high reflectivity and a desired mosaicity have been unsuccessful. Moreover, it was generally viewed as not possible to produce such slow neutron Ge crystal monochromators.